1. Field of the Invention
This invention relates to a crystal growing device and method, in which a liquefied body with the composition of the crystal to be formed is enclosed in a crucible of which it wets the wall and left to solidify in the presence of a seed that has the crystalline structure to be created, so that this structure is reproduced by the body as it solidifies. In an embodiment of the method, an ingot of the body is deposited in the solid state in the crucible and it is liquefied by stages by a movable means of heating along the crucible while moving away from the seed: the body solidifies progressively from the seed, from which it again takes the crystalline structure. The original body thus transformed into a crystal is then recovered by opening the crucible. Several concrete examples of these methods have been conceived in industry.
2. Description of Related Art
French patent 2 757 184, which constitutes the nearest prior art, teaches that the solidification of the crystal causes certain difficulties since its coefficient of contraction is different to that of the crucible when it cools down, which produces mechanical stresses, defects in the crystals and sometimes a rupture of the crucible, even if it contracts less than the crystal, since it adheres to it. This patent also teaches that a solution may be found by producing a higher gas pressure in a bottom part of the crucible, where the crystal forms, than in an upper part of the crucible, which overhangs the liquid and is thus occupied by the gas. This overpressure automatically produces a contraction of the section of the crystal at the moment of its solidification, and thus a gap between the crystal and the crucible, which increases with the level of the crystalxe2x80x94liquid interface as the solidification progresses.
This invention constitutes an improvement to the previous patent: although the known process enables the asserted objectives to be attained, it has certain disadvantages due to the existence of pipes leading to the two extreme regions of the crucible and which make it possible to establish the desired pressure difference. It must first of all be admitted that this grafting of a gas circuit onto the crucible imposes modifications to the apparatus and somewhat perturbs the heat exchanges: but above all, one observes condensations of the body that should form the crystal in the cold parts of the pipes when these bodies have a high vapour pressure, such as CdTe, GaAs, InP, GaP in particular. The composition of the crystal may be affected by this, and the pipes end up by becoming blocked. It would be conceivable to maintain the whole of the gas circuit at a sufficient temperature to preclude the condensation of these bodies, but the technical difficulties would be considerable due to the high temperature involved (1000xc2x0 C. for example), and there is no pressure sensor, which is however necessary to adjust the pressure difference, which would resist such heat.
An essential aim of the invention is thus to do away with the gas circuit without doing away with the beneficial effect of the pressure difference between the two parts of the crucible. Henceforth one uses a crucible that is sealed but fitted with a gas pocket, called ampoule, heated by a specific means and adjustable, which makes it possible to modify the temperature of the gas at will. The ampoule communicates with the bottom part of the crucible and there establishes the desired overpressure via the heating of its contents. Thus, a pressure regulation is replaced by a temperature regulation, which is easier to accomplish in practice. A visual observation of the solidification and, in particular, the crystalxe2x80x94liquid interface makes it possible to ensure the overpressure is correct without having to resort to a sensor.